Vibration-damped structure



April 1963 G. KURTZE 3,087,568

VIBRATION-DAMPED STRUCTURE Filed March 23. 1959 IN VEN TOR.

UEN Tl-IER KURTZE A T'TOIFNEV United States Patent 3,687,568 VIBRATION-DAMPED STRUCTURE Guenther Kurtze, Arlington, Mass, assignor to Bolt Beranek and Newman, inn, Cambridge, Mass, a cerporation of Massachusetts Filed Mar. 23, 1959, Ser. No. 301,197 7 Claims. (Cl. 181-33) The present invention relates to vibration-damped structures and, more particularly, to damping with the aid of viscous media.

Damping of structure-borne sound vibrations is usually obtained by covering the structural elements with layers or coatings of sound-dissipative materials. in this way, energy is withdrawn from the sound-vibration waves by irreversible deformations of these coatings. The terms sound and vibrations, as herein employed, are intended to connote generically all types of elastic vibrations and the like, whether audible, sub-audible or superaudible.

The present invention, on the other hand, is more particularly concerned with the damping of double-walled structures and the like, and involves the use of viscous materials as dissipative media.

An object of the invention is to provide a new and improved vibration-damped structure.

A further object is to provide for the damping of double walls, partitions, panels, surfaces and the like, all hereinafter referred to by the generic term plates and the like.

Still an additional object is to provide a novel apparatus for damping vibrations traveling along any pair of vibration-wave propagating paths, whether of plate configuration or of other configuration for permitting the passage of sound or vibration waves therealong.

Other and further objects will be discussed hereinafter and will be more particularly pointed out in the appended claims.

The invention will now be described in connection with the accompanying drawing, FIG. 1 of which is a transverse section of a damped double-plate structure constructed in accordance with the present invention; and

FIG. 2 is a similar view of a preferred modification.

Referring to the drawing, the invention is illustrated as applied to a double-plate structure, though it is to be understood that the teachings hereof may equally well be applied to other pairs of vibration-propagating pathstructures, including rods, tubes, pipes and the like, to mention but a few. The plates 1 and 3 are shown generally co-extensive and of somewhat dilferent thickness so that the vibration-propagation or bending-wave velocity along the plates, say from one end to the other, is substantially different; in this case, the velocity C along the plate 1 is faster or greater than the velocity C along the plate 3. Different velocities can also be obtained in other ways, as by using different materials, diiferent temperatures, and other techniques.

Where different bending wave velocities are involved, there will be phase shifts between the bending-wave motion of the two plates which increase with increasing distance from the source end, resulting in relative motions of the two plates with respect to each other. If these two plates 1 and 3 are coupled over a substantial portion of the region separating them by a viscous medium 5, as of viscous liquid, plastic, semi-solid ma terial, etc., an alternating flow will be generated by this relative motion, and the viscosity of the medium 5 will give rise to energy losses.

It will be observed that losses do not occur in the theoretical limiting cases of zero and infinite viscosity, since zero viscosity means no dissipation, and infinite 3,087,568 Patented Apr. 30, 1963 ice given by ps1 is %;2 d1vI2 Z pd pd (1) and the loss tangent at this frequency is =density of the viscous medium 5, d=thickness of the viscous medium 5, n=viscosity of the medium 5,

M =mass per unit area of plate 3, and M bending wavelength in plate 1.

Equation 2 shows that the maximum obtainable damping its proportional to 1/) and hence decreases with decreasing frequency. This may be disadvantageous when damping at low frequencies is desired. For such cases,

where the disadvantage can be obviated, in accordance with the invention, by partitioning or subdividing the medium 5 with partitions, such as, for example, foam plastic separators 7, FIG. 2, with closed, air-filled cells 9. A representative embodiment employed steel plates 1, 3 that were each 1 cm. thick, but with stiffness differing by a factor of ten, and a liquid medium 5 of silicon oil 1 cm. thick and of viscosity ,u 668.

The partitions 7 are disposed at intervals I only along one direction (shown longitudinally spaced), the distance or separation I being much less than the wavelengths of the band of frequencies to be damped. The partitions 7 may also be of other materials, though they must have the property of being soft or highly compressible compared With the viscous liquid or other medium 5 in order to decouple adjacent medium regions from one another.

The construction of FIG. 2 yields a wider or broaderband frequency range of effective damping than for the construction of FIG. 1. Assuming again, that c 2c the frequency w of maximum damping is given substantially by p and the maximum obtainable loss tangent is P p N WI; 12am) According to Equation 4, 7; no longer depends on frequency so that the same amount of maximum damping can be obtained at any frequency by proper choice of [.L in Equation 3.

If the plates 1 and 3 are made of light-transmitting material, such as transparent or translucent glass and the like, and the medium 5 is also light-transmitting, such as, for example, a clear mineral oil of suflicient viscosity, a sealed unit 1, 3. 5 could well serve as a vibrationdamped window or the like.

Further modifications will occur to those skilled in a the art and all such are considered to fall within the spirit and scope of the invention as defined in the appended claims. The terms steel-like and liquid-oillike as employed in the claims are intended to embrace materials which are like steel or like liquid oil, respectively, in the function performed by the designated elements in the panel of the invention. For example, as stated above the spaced layers may be formed of gl-ass, which is steel-like as to stiffiness and rigidity when compared to a viscous liquid-oil-like material, including the oils, plastics, or semi-solids described.

What is claimed is:

1. An acoustic panel for damping a predetermined acoustic frequency f with a maximum degree of damping and frequencies above and below said predetermined frequency with substantial, but lesser damping, comprising a pair of layers of stiff steel-like sheet material spaced apart, and a body of viscous liquid-oil-like material between and continuously contacting said layers, both said layers being much more rigid than said viscous material, one of said layers having an overall bending stiffness substantially greater than the overall bending stiffness of the other layer, whereby the acoustic bending wave propagating velocity C of one layer is substantially greater than the acoustic bending wave propagating velocity C of the other layer and the phases of acoustic vibrations along said layers are different, said panel having maximum damping at said predetermined acoustic frequency determined substantially by the relationship:

drag/9 3 1%) a) 11M, 4 d d 2 where w:21rf, p is the densityof the viscous material, d is the thickness of the viscous material, p. is the viscosity of the viscous material, and M is the mass per unit area of the slower-velocity layer.

2. The panel of claim 1, said sheet material and said viscous material being light-transmitting.

3. The panel of'claim 1, said layers having different thicknesses.

4. An acoustic panel for damping a predetermined acoustic frequency f with a maximum degree of damping and frequencies above and below said predetermined frequency with substantial, but lesser damping, comprising a pair of layers of stiff steel-like sheet material spaced apart, a body of viscous liquid-oil-like material between and continuously contacting said layers, both said layers being much more rigid than said viscous material, one of said layers having an overall bending stiffness substantially greater than the overall bending stiffness of the other layer, whereby the acoustic bending wave propagating velocity 0 of one layer is substantially greater than the acoustic bending wave propagating velocity 0 of the other layer and the phases of acoustic vibrations along said layers are different, and a plurality of spaced partitions extending between said layers and dividing said viscous material, the compressibility of said partitions being substantially greater than that of said viscous material, said panel having maximum damping at said predetermined acoustic frequency determined substantially by the relationship:

,, pd2 1203M References Cited in the file of this patent UNITED STATES PATENTS 1,193,013 Grant Aug. 1, 1916 2,407,400 Chamberlain Sept. 10, 1946 2,724,670 Mason Nov. 22, 1955 2,850,109 Benjamin Sept. 2, 1958 FOREIGN PATENTS 754,299 Great Britain Aug. 8, 1956 1,135,422 France Dec. 17, 1956 OTHER REFERENCES Cyril M. Harris: Handbook of Noise Control (Mc- Graw-Hill Book Company, Inc., New York, 1957), pages 12-1, 12-2, 12-8 through 12-10 and 14-13 through 14-18.

S. Timoshenko: Vibration Problems in Engineering (D. Van Nostrand Company, Inc., New York, 1955), pages 210-220. Third edition. 

1. AN ACOUSTIC PANEL FOR DAMPING A PREDETERMINED ACOUSTIC FREQUENCY F WITH A MAXIMUM DEGREE OF DAMPING AND FREQUENCIES ABOVE AND BELOW SAID PREDETERMINED FREQNENCY WITH SUBSTANTIAL, BUT LASSER DAMPING, COMPRISING A PAIR OF LAYERS OF STILL STEEL-LIKE SHEET MATERIAL SPACED APART, AND A BODY OF VISCOUS LIQUID-OIL-LIKE MATERIAL SPACED TWEEN AND CONTINUOUSLY CONTACTING SAID LAYERS, BOTH SAID LAYERS BEING MUCH MORE RIGID THAN SAID VISCOUS MATERIAL, ONE OF SAID LAYERS HAVING AN OVERALL BENDING STIFFNESS SUBSTANTIALLY GREATER THAN THE OVERALL BENDING STIFFNESS OF THE OTHER LAYER, WHEREBY THE ACOUSTIC BENDING WAVE PROPAGATING VELOCITY C1 OF ONE LAYER IS SUBSTANTIALLY GREATER THAN THE ACOUSTIC BENDING WAVE PROPAGATING VELOCITY C2 OF THE OTHER LAYER AND THE PHASES OF ACOUSTIC VIBRATIONS ALONG SAID LAYERS ARE DIFFERENT, SAID PANEL HAVING MAXIMUN DAMPING AT SAID PREDETERMINED ACOUSTIC FREQUENCY DETERMINED SUBSTANTIALLY BY THE RELATIONSHIP: 